Lockup TCC Wiring

This page is all about what I learned about the wiring needed to control the
lockup torque convert clutch (TCC) after I decided to swap the original
TH400 transmission in my 1975 GMC
Suburban for a TH700R4 transmission to
gain the benefits of a modern
overdrive transmission and help tame my "great for towing" 4.11 rear axle gears
on the highway. As part of this project, I also had to build a
custom Throttle Valve Cable
Bracket so I could hookup the TV cable to the carburetor. This information
applies in large measure to the TH200R4 transmissions, but not to any of the
computer-controlled transmissions that came later - they are all controlled by
the computer and the wiring is more or less just to all the sensors involved and
is specific to the computer you have to use.

Overview

The only wiring work you need to do on these transmission is wire up the controls for the lockup feature
on the torque converter. This can be accomplished with simple kits from several
places or with custom wiring of your choice. The kits typically have two wires
going into the transmission - one is power to the
torque converter clutch and another is a ground connection. When you have power
and a ground, the converter is "locked". When one or both are disconnected, the converter
is unlocked. The kits typically have a vacuum switch, some basic wiring,
perhaps a pressure switch to know what gear the transmission is in, and
sometimes even a simple speed sensor to prevent lockup below a certain MPH. These kits are all "plug and play" simple,
so you can just buy one and be done with it. Or you can figure it out and create
your own kit - it's your choice and your money. Being the sort of "gotta do it
my way" kind of guy that I am, I decided to do my wiring from scratch and figure
out all the details along the way.

One thing of note is that some of the wiring is inside the
transmission pan, so when you drain any remaining fluid by removing the pan to
change the filter,
that's the time to get the wiring done the way you want it. No sense doing that
more than once.

Details

Here are some pictures of the wiring on my donor transmission as I received
it. The first photo shows the wiring connector on the driver's side of the
transmission with the three clipped wires coming out of it - look closely, it is
there in the grime and dirt. It shows the three wires coming out of the wiring plug on the
transmission and the basic location of the wiring plug in relation to the shift
linkage. The next four photos show the internal wiring inside the transmission
pan - there was quite a bit inside here on my donor transmission! The final
photo shows the underside of the transmission case connector - the white plastic
piece is clipped into the transmission housing and there are plus that go into
it from the top and bottom - it's quite a complicated little piece!

It's important to know what all that wiring does and why it's there - because
I'm going to change it to suit the needs I have after swapping this transmission
into a non-computer-controlled vehicle. There are a total of four possible
pressure switches, three possible wires coming into the transmission, and a
solenoid to do the actual work of locking up the torque converter. Note that I
said "possible" - not all transmissions used all of the pressure switches and
wiring. In addition, there are two different style solenoids that were used in
various applications.

For reference, I'm including a pretty detailed list of the wiring and
switches inside my transmission. It had pretty much every possible wiring option
and folks can use this to help ID what goes where for doing retrofit or swap
applications. I had a heck of a time figuring this out from available
information on the internet and eventually had to buy a factory manual for the
year and model vehicle my donor transmission came from to confirm all the
details. I'm posting it here for others to learn from my experience.

The solenoid has the single red wire going to it and is next to the
pressure switch with the single green connector. This solenoid is the single
wire "self-grounding" style solenoid. Most swap applications I've seen
(including kits from TCI, etc.) require using the two wire style solenoid
with a separate ground wire that can be controlled - it's easier to do the
wiring on the ground side of things in many situations.

The pressure switch with the single green connector is installed in the
"TCC Signal" location and is a normally open unit that is self-grounding. I
believe the contact is connected to ground whenever the TCC is engaged.

The pressure switch with the two blue connectors on it is installed into
the "4th Clutch" location and is a normally open unit that is not grounded.
The contacts are connected whenever the transmission is in 4th gear.

The pressure switch with the two green connectors on it is installed
into the "4-3 Downshift" location and is a normally closed unit that is not
grounded. I believe that the contacts are disconnected during the 4-3
downshift and I would assume this prevents problems with the TCC being
locked up during the downshift.

The pressure switch with the two red connectors on it is installed into
the "3rd Clutch" location and is a normally open unit that is not grounded.
The contacts are connected whenever the transmission is in 3rd gear.

The diagram below is what I believe to be correct for my transmission, though
the wiring terminal colors they list don't match what I found inside my
transmission. That could be due to my transmission being rebuilt at some time,
or due to the manual not matching what actually happened on the assembly line.
Either way, the details are handy - especially the small wiring schematic in the
upper right hand corner of the page and the listing of what wires are A, B, C,
and D in the transmission case connector. Note that terminal C is not used in
any application that I can find and my transmission electric plug had a solid
plug in that position so it was not usable. It might be nice if it was there to wire
a simple "TCC is engaged" light on the dash that was not a "I think it should be
engaged", but rather "it really is engaged" switch...

If you choose to not use any of the switches, they can be left in place, or
removed and replaced with 1/8" NPT plugs. All four holes must have a switch or
plug in them for the transmission to function, so double check stuff or you'll
be removing the transmission pan and fixing it once the transmission is in the
car - quite the messy experience. I opted to
remove all but the "4th Clutch" switch and replace the others with 1/8" NPT
plugs - the switches seemed to work and were nifty enough to save for other
possible projects. Plus, removing the "TCC Signal" switch makes it easier to get
at one of the bolts for the solenoid, which I had to remove to see about changing to the
two-wire style solenoid from my original one-wire self-grounding style solenoid.

Here's the TCI internal wiring diagram for reference. Note that is shows pipe
plugs in the three unused pressure switch locations. The labeling is done in a
somewhat confusing way, but it does contain useful information, so I'm including
it here for reference purposes.

Here was my first cut at a wiring diagram that should work similar to the TCI
kit, plus this will have an indicator light and the ability to force the torque
converter to lock up in any gear. Like most other kits, it controls everything
on the ground side of the system, though the control switch does toggle things
completely off when in the center OFF position. I like having that control, but
it's not strictly needed. It does offer a nice safety feature in case of
electrical system problems down inside the transmission, so I would prefer to do
it this way. If this added niftiness isn't important to you, it's possible to
substitute a SPDT center-off switch and run the power wire straight out to the
solenoid with a wire heading up to the LED indicator as shown here. Since SPDT
center-off switches might be easier to find than DPDT center-off switches this
could be a viable option for some folks.

If you have the older GM style cruise control where the power goes
through the brake switch first and then out to the cruise system, the cruise
control will still work if you simply splice into the existing switch wiring
as shown. If you have a newer style system, then you will need to add a
second switch for the TCC or wire up a suitable relay system to drive things
properly.

The indicator LED will likely need a dropping resistor of the proper
size for whatever LED you choose and the overall brightness you want. You
could even get tricky here and rig up something that would make the LED less
bright when the dash lights were on, but that's a different topic entirely
and would add some extra complexity. Not too hard, so I'm just tossing it in
here as a thought.

The vacuum switch in the "TCC Automatic Lockup" wiring ensures that the
TCC unlocks in situations of low or no vacuum. Since it's hooked to a ported
vacuum source, this means the switch is open at idle and when the engine is
under heavy load - exactly when you want the TCC to be unlocked. The factory
switches I've seen have a small vacuum restrictor/delay valve inline just
before the actual switch, and this helps delay lockup until you have a few
seconds of good, stable vacuum going to the switch. So long as the vacuum
rating of the switch and the delay time on the valve are reasonably matched
to your engine combination, this should result in the effect of only locking
up the TCC when the engine is under stable operation. TCI's kit uses an
adjustable vacuum switch to enable you to fine-tune this.

The brake switch makes sure that the TCC is unlocked when you step on
the brake - this is important in "panic stop" situations where you jam on
the brakes and might lock the tires. It also ensures that the TCC is off
when you at sitting at a stoplight.

After creating that basic diagram and pouring over the manuals, I found a few
interesting details that kept my idea hamster going for a while. This is more
stuff to think about before deciding on what wiring to use for your
transmission.

The original wiring diagrams show a "4-3 Downshift Switch" that on my
transmission was a normally closed switch. The shop manual does not speak to
this, but based on the wiring and what details I can find, it seems to
indicate that this switch opens during the 4-3 downshift, and thus unlocks
the TCC in this case. I can't decipher the timing on this to know if it
matters to wire the 4-3 downshift switch in series with the 4th gear switch
in an retrofit application, but if it got the TCC to unlock a touch sooner,
then it might be interesting, useful, and simple to add this into the wiring
for the "TCC Automatic Lockup" wiring. If you did this, you could splice the
"4-3 Downshift Switch" into the green wire that goes between the 4th Gear switch and
terminal D on the transmission case connector. Or you could splice it in
between the 4th Gear Switch and the solenoid to have it act in both
positions of the dash switch. This is provided mainly as food for thought -
I'm not sure enough to say what exact effect this would have once the
vehicle was being driven. If someone knows how this switch actually works inside the transmission and under
what conditions this switch would be activated, feel free to
drop me a line and explain it so I can post the details here.

The "TCC Signal Switch" appears to be a self-grounding normally open
pressure switch that makes contact to ground whenever the TCC hydraulic
system has pressure. If so, this is a much more interesting way to hook up
the TCC Lockup Indicator. In the wiring above, the indicator light really
means "we have applied electrical power to the TCC lockup solenoid", not
"the TCC is locked up". If this switch does what I think it does, it could
be used to drive the indicator light directly, in which case the indicator
light would mean "the TCC lockup system has hydraulic pressure applied to
it" - a much more useful piece of information if something goes awry in the
future, such as the solenoid crapping out or the TCC hydraulic system having
problems. The indicator light would reflect the reality inside the
transmission much more directly. As with the "4-3 Downshift Switch" above,
this is more food for thought, and if someone knows how this actually works
inside the transmission and under what conditions this switch would be
activated, feel free to
drop me a
line and explain it so I can post the details here.

There are only three wires that go into the transmission case connector,
even though it looks to have provisions for four. The case connector on mine
has a solid plug in the "C" location, thus preventing use of the fourth
terminal. This presents a problem if you want to try and use the "TCC Signal
Switch" idea above, because the original wiring diagram already uses all
three connections. You can either dump the "force TCC lockup in any gear"
feature, or do some creative rewiring to move the control to the positive
side of the system. This requires the use of a one-wire self-grounding
solenoid, but other than that and some wiring differences, it's basically
the same.

The "TCC Always On" wiring in the diagram above really means "always on"
- the TCC always on unless you step on the brakes. That means you could get
an unpleasant surprise if the TCC is locked and you step on the brakes, stop
the vehicle, then let off the brakes. At that point the TCC will try to
engage wit the vehicle stopped, running, and in gear. The engine will likely
stall or at least buck as the TCC will try to lock even though you are not
moving yet. "ON" really does mean "ON" in this case! One idea to fix this is
to rig up a low-speed switch of some kind to open the circuit below, say, 10
MPH. It would be interesting to wire it into this system and see how it
worked. I'd bet acceleration would be sluggish, and the TCC could have
longevity issues (just theorizing here, no hard proof), but it would
certainly be a nice safety feature for when others might drive your vehicle
- aka, the wife. Don't say you weren't warned!

Another route to "idiot proof" the system would be to wire the vacuum
switch inline with the power wire so that the brake switch and the vacuum
switch had to be closed to allow the TCC to engage. That would make the "TCC
Always On" mean "TCC On In Any Gear" and the auto-unlock under load would
still work, which makes a bit more sense.

Always wiring in the vacuum switch has the side effect of removing the
possibility of getting the TCC locked while coasting down a long grade
without touching the accelerator pedal. In that situation, ported vacuum is
non-existent, manifold vacuum is high, and the brakes might even be applied
as well. Because you need to hook the vacuum switch up to ported vacuum to
prevent engagement at idle, in the coasting situation this would open the
switch and unlock the TCC. Also, if you hit the brakes, you would also
remove power and unlock the TCC. One idea here would be another vacuum
switch that would allow the TCC to stay locked as long as manifold vacuum
was very high. To do this, it would be bypassing the brake switch and the
vacuum switch to provide power directly to the main TCC lockup control
switch and like the brake switch and the other vacuum switch, it would be a more-or-less automatic thing.
A switch that would work for this is available from
PATC under part number
71V2. It has normally open and normally closed
contacts, and is adjustable from 6 to 22 inches of vacuum. You would wire it
through the normally open contacts, hook it up to manifold vacuum, and
adjust the vacuum level so it activated the switch around 18 inches of
vacuum - or whatever worked best for your vehicle. The idea would be that as
you let off the gas, if you go dead-throttle and the engine is being used
for "engine braking", the manifold vacuum will rise sharply. My adjusting
the cut-in point for this and using the proper delay valve on the other
vacuum switch, it should be possible to dead-throttle the vehicle on a
grade, and even hit the brakes, but have the TCC stay locked up until
manifold vacuum dropped back down again, aka, the grade levels out, you hit
the gas again, or you slow down enough that the engine revs come down. The
switch is a hefty $43, but experimenting is pretty easy to do and the wiring
hookups are pretty simple.

Sticking with the ground side controls, here's a wiring diagram the moves the
vacuum switch into the main power line. It's a bit odd because the wire coming
off the 4th Gear Pressure Switch simply exits the transmission case and goes to
ground, but it should work fine, and doing it this way allows the indicator to
work the same way it does in the previous diagram. You could in theory substitute a self-grounding
switch here as well and only have two wires coming out of the transmission case,
but I'm not sure at what pressures the various switches trigger at. Because of
that, I would tend to use them in their original locations to be sure all was
working as expected/desired.

Taking things a step further, I created this diagram to move the control
completely to the power side, switch to a one-wire self-grounding solenoid, and
try to use the self-grounding switch in the "TCC Signal" location to control the
indicator light. If this works, I think the indicator light would be more useful
- assuming, of course, that my understanding of the "TCC Signal" switch is
correct. If not, the previous diagram is probably the best so far. I have also
included the required changes to have the vacuum switch control the power feed
to the dash switch.

This is a small update on the previous diagram to add a "very high vacuum"
switch to allow the TCC to stay locked whenever manifold vacuum is very high -
such as when doing engine braking on a long downgrade. Other than a bit of
shuffling on the diagram to make everything fit neatly, the only addition is the
vacuum switch connected between the power side of the brake switch and the power
feed to the SPDT Center-Off switch. This switch is hooked up to manifold vacuum
(the other is hooked to ported vacuum) and can be tapped off the same vacuum
line that goes to your vacuum gauge - you do have one of those, don't you? - so
that you don't have to run more wires out into the engine compartment. If you
decided to wire this new switch in the engine compartment, you would need a
third wire heading out to the new vacuum switch and the two vacuum switches
could share a common return wire back to the switch. This addition can easily be
done after the above wiring has been done and tested.

Finally, a bit of re-arranging and cleaning things up netted me this diagram.
Before creating this, I roughed in the wiring on the vehicle and made this
diagram follow the rough harness layout to make it easier to conceptualize what
goes where while I was staring at the diagram and doing the actual wiring. On
the actual harness I added connectors just inside of the firewall and just
before the switch so the harness could be built in pieces outside the vehicle.
If I had been able to size it better beforehand, these extra connectors would
not have been needed, but they made my life easier and will come in handy if I
ever have to remove or disconnect the wiring for service or troubleshooting.

Also, after playing with the wiring a bit on the vehicle while driving
around, I could not get the indicator light to work the way I originally thought
it should. To make the light work more sensibly, I added a ground wire to ground
the indicator light and ran the "power" side of the light from the "lockup in
any gear" lead coming off the switch. This lead is connected to the solenoid
power lead inside the transmission, and gets power when the solenoid has power
applied to it. That makes the indicator light go on whenever the solenoid should
be locking up the converter. I'll have to research the pressure switch I was
trying to use a bit more.

Here are the photos of the wiring work I did as I put things back together.

Wire Connectors

Here are close-up shots of the various wire connectors used on the TH700R4
transmission. The connectors on the pressure switches were standard fare - I had
a box of crimp-on connectors already, so I just pulled the old wires out of the
plastic connectors and put in new wires with new crimp-on connectors. I had a
heck of a time finding the right connectors for the internal wiring plug, but I
did manage to get a few that seem to work fine at my local GM dealer. He did
have to scrounge through his wiring terminal kit to find them, though. The
external wiring connector I got from the dealership has wiring pigtails attached
and came with crimp-on butt connectors, but you can get the bare connector with
terminals from PATC under
part #71XXX. The case connector is still available from GM if needed, complete
with a new o-ring. Check out my
Automatic Overdrive Transmission Swap page for a complete list of all of the
relevant parts, part numbers, and sources.

TODO: Update this page once I have done some road testing of my
transmission and lockup wiring!

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